2. Anaemia

One of the most common medical reasons for feeling constantly run down is iron deficiency anaemia. It affects around 1 in 20 men and postmenopausal women, but may be even more common in women who are still having periods.

Typically, you'll feel you can't be bothered to do anything, your muscles will feel heavy, and you'll get tired very quickly.

Women with heavy periods and pregnant women are especially prone to anaemia.

3. Chronic fatigue syndrome

Chronic fatigue syndrome (myalgic encephalomyelitis, or ME) is a severe and disabling tiredness that goes on for at least six months. There are usually other symptoms, such as a sore throat, muscle or joint pain, and headache.

6. Diabetes

One of the main symptoms of diabetes, a long-term condition caused by too much sugar in the blood, is feeling very tired. The other key symptoms are feeling very thirsty, going to the toilet a lot, and weight loss. Your GP can diagnose diabetes with a blood test.

8. Depression

As well as making you feel very sad, depression can also make you feel drained of energy. It can stop you falling asleep or cause you to wake up early in the morning, which makes you feel more tired during the day.